Ultrasonic imaging with acoustic resonant cavity

ABSTRACT

Techniques describe structures and methods for generating larger output signals and improving image quality of ultrasonic sensors by inclusion of an acoustic cavity in the sensor stack. In some embodiments, an ultrasonic sensor unit may be tuned during manufacturing or during a provisioning phase to work with different thicknesses and materials. In some embodiments, a standing wave signal may be generated using an acoustic cavity in the ultrasonic sensor unit for capturing an ultrasonic image of an object placed on a sensor surface. In some implementations, the ultrasonic sensor may include an ultrasonic transmitter, a piezoelectric receiver, a thin film transistor (TFT) layer and a TFT substrate positioned between the transmitter and the receiver, one or more adhesive layers, and optional cover materials and coatings. The thickness, density and speed of sound of the sensor materials and associated adhesive attachment layers may be used to attain the desired acoustic cavity and improved performance.

CROSS REFERENCE SECTION

This application is a continuation of U.S. Non-Provisional applicationSer. No. 14/589,783, filed Jan. 5, 2015, titled “ULTRASOUND IMAGINGUSING RESONANT CAVITY,” which claims the benefit and priority of U.S.Provisional Application No. 61/926,829, filed on Jan. 13, 2014, titled“ULTRASOUND IMAGING USING RESONANT CAVITY,” which is herein incorporatedby reference in its entirety for all purposes.

TECHNICAL FIELD

The present disclosure generally relates to sensing technology and morespecifically to ultrasonic sensing technology.

BACKGROUND

Sensors detect physical input and in some instances convert the physicalinput to electrical or optical output. A device may use the electricaloutput in a variety of ways. Applications of sensors are widespread andsensors are used in everyday devices, such as mobile devices.Specifically, sensors are gaining popularity for biometric readings,such as for reading fingerprints and for authenticating users.Traditionally, ultrasonic systems have operated by interpreting theechoes from sound waves reflecting off of distant objects. For instance,ultrasonic sensors may generate high-frequency sound waves and evaluatethe echo that is received back by the sensor. Ultrasonic sensorsgenerally calculate the time interval between sending the signal andreceiving the echo to determine the distance to an incident surface ofthe distant object. In some implementations such as mobile phones,tablet computers, wearable health-monitoring devices and other mobiledevices, the total thickness for the ultrasonic sensor must be small, onthe order of one millimeter thick or less, limiting the use oftraditional approaches.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the disclosure are illustrated by way of example. Thefollowing description is provided with reference to the drawings, wherelike reference numerals are used to refer to like elements throughout.While various details of one or more techniques are described herein,other techniques are also possible. In some instances, well-knownstructures and devices are shown in block diagram form in order tofacilitate describing various techniques.

A further understanding of the nature and advantages of examplesprovided by the disclosure may be realized by reference to the remainingportions of the specification and the drawings, wherein like referencenumerals are used throughout the several drawings to refer to similarcomponents. In some instances, a sub-label is associated with areference numeral to denote one of multiple similar components. Whenreference is made to a reference numeral without specification to anexisting sub-label, the reference numeral refers to all such similarcomponents.

FIG. 1 illustrates buildup of a standing wave signal according to one ormore aspects of the disclosure.

FIG. 2 illustrates a cross-sectional view of an example configurationfor an ultrasonic sensor unit.

FIG. 3 illustrates a flow diagram for performing a method according toone or more aspects of the disclosure.

FIG. 4A and FIG. 4B respectively illustrate two graphs representing theformation of a standing wave signal with varying amplitude in anacoustic cavity of an ultrasonic sensor unit.

FIG. 5 illustrates a cross-sectional view of an example ultrasonicsensor unit.

FIG. 6A illustrates a graph showing the frequency response of anexemplary sensor output with different numbers of excitation signalpulses (or cycles) for an example implementation of an ultrasonic sensorunit.

FIG. 6B illustrates the increase in the response and then a leveling offof the resonance amplitude with an increasing number of excitationsignal pulses (or cycles) for the acoustic cavity.

FIG. 6C illustrates successive improvement in the image quality of afingerprint image as the number of excitation signal pulses areincreased and saturation of the acquired image is approached.

FIG. 7A shows an example top view of an implementation of an ultrasonicfingerprint sensor.

FIG. 7B shows an example side view of the ultrasonic fingerprint sensorarray.

FIG. 8 illustrates a cross-sectional view of another exampleconfiguration for an ultrasonic sensor unit.

FIG. 9 shows an example ultrasonic sensor with a cover layer above thethin film transistor (TFT) substrate and receiver (Rx).

FIG. 10A and FIG. 10B illustrate graphs showing the frequency responseof an example ultrasonic sensor, with different numbers of excitationsignal pulses (or cycles) for two different thicknesses of polycarbonatecover layers.

FIG. 10C illustrates differences in sensor output voltages at theresonant frequencies for the sensors represented in FIG. 10A and FIG.10B, with respect to the different number of excitation signal pulses.

FIG. 11A illustrates an exploded view displaying various layers of amaterial stack for an example ultrasonic sensor unit with an acousticcavity resonator.

FIG. 11B illustrates an assembled view of an example ultrasonic sensorunit with an acoustic cavity.

FIGS. 12A, 12B, 12C and 12D illustrate various example configurationsand placements of an example ultrasonic sensor unit with a display orcover glass of a mobile device.

FIG. 13 illustrates a block diagram for an example representation of anultrasonic sensor unit.

FIG. 14 illustrates an example of a computing system in which one ormore embodiments may be implemented.

SUMMARY

Aspects described herein provide structures and methods for generatinglarger output signals and improving image quality of ultrasonic sensorsby inclusion of an acoustic cavity in the sensor stack of an ultrasonicsensor unit, allowing the amplitude of the generated ultrasonic wave toincrease with multiple excitation signal pulses compared to approacheswhere the amplitude of the generated wave is limited by the displacementattainable with single-cycle excitation. In some aspects, the thicknessof the ultrasonic sensor unit may be significantly reduced toaccommodate the low profiles often desired for mobile devices. In someimplementations, the ultrasonic sensor may be designed and fabricated toform an effective acoustic cavity for the desired operating frequencieswith the various substrate materials, electrode materials, adhesives,piezoelectric materials, and other materials (e.g. cover glass, platen,cover layer, coatings, etc.) that may be desired. The ultrasonic sensormay be tuned or otherwise calibrated during manufacturing or during aprovisioning phase to work with different encapsulations, thicknessesand materials.

In some implementations, an ultrasonic standing wave signal may begenerated using an acoustic cavity in the ultrasonic sensor forcapturing an ultrasonic image of an object placed on an imaging surfaceof the sensor. In some implementations, the ultrasonic sensor may havean ultrasonic transmitter (Tx) including a piezoelectric transmitterlayer and one or more transmitter electrodes disposed on opposing sidesof the piezoelectric transmitter layer, a receiver (Rx) including apiezoelectric receiver layer with a receiver bias electrode disposed onone side of the piezoelectric receiver layer, and a thin film transistor(TFT) layer disposed on a TFT substrate that may be positioned betweenthe transmitter, receiver and any associated adhesive attachment layers,cover layers or coatings. In some implementations, the thickness of andspeed of sound within the piezoelectric transmitter and receiver layersand the Tx and Rx electrodes along with the adhesive layers, the TFTsubstrate and other layers may be selected and used to form the desiredacoustic cavity.

In some implementations, a standing wave signal may be built up bylaunching multiple cycles (e.g. 4-8) of single- or double-digitmegahertz ultrasonic waves. The acoustic cavity designed into the sensorstack allows buildup of the amplitude and acoustic energy of theultrasonic wave within the cavity prior to acquiring an image. Theresonant frequency of this acoustic resonant cavity may be predominatelydetermined by the thickness of individual layers and total thickness ofthe stack, the density of each material in the stack, the elastic moduliof materials in the stack, the speed of sound in each material, and therigidity of the boundary conditions. The resonance may bedamped/enhanced or shifted when an object (such as a finger) is placedon the sensor surface. Moreover, the amplitude and/or phase of thereflected signal may be different with and without an object positionedon the surface. As a result, changes in the magnitude and/or phase ofthe standing wave may be detected by measuring the generated sensoroutput voltages at the receiver and converting the sensor outputvoltages to digital information with, for example, an analog to digitalconverter (ADC).

An example method for generating an image of a target object may includeapplying a plurality of excitation signal pulses to an ultrasonictransmitter of an ultrasonic sensor unit, wherein a frequency of theplurality of excitation signal pulses is selected to generate anultrasonic standing wave signal inside the ultrasonic sensor unit andwherein the plurality of excitation signal pulses are applied for aduration to allow buildup of energy for the ultrasonic standing wavesignal over a first threshold level; detecting a change in one or morecharacteristics of the ultrasonic standing wave signal associated withan interaction between the ultrasonic standing wave signal and thetarget object using an ultrasonic receiver of the ultrasonic sensorunit; and generating the image of the target object based on thedetected change in the one or more characteristics of the ultrasonicstanding wave signal.

In certain aspects, the duration may be based on passing of a referencetime period or the number of the excitation signal pulses. The number ofexcitation signal pulses may include 4 pulses or more, 5 pulses or more,or 6 pulses or more. Detecting the change in the one or morecharacteristics of the ultrasonic standing wave signal may includedetecting a change in amplitude of the ultrasonic standing wave signal,a change in phase of the ultrasonic standing wave signal, or both.

In certain aspects of the method, the ultrasonic standing wave signalmay result from constructive interference of generated ultrasonic wavesignals with one or more reflected ultrasonic signals inside an acousticcavity of the ultrasonic sensor unit at the selected frequency. The oneor more ultrasonic reflected signals may be generated from reflection ofthe ultrasonic wave signals off of one or more boundaries of theultrasonic sensor unit.

In certain implementations of the method, the change in the one or morecharacteristics of the ultrasonic standing wave signal may be detectedwhile the excitation signal pulses are being applied to the ultrasonictransmitter of the ultrasonic sensor unit. In another implementation ofthe method, the change in the one or more characteristics of theultrasonic standing wave signal may be detected after the excitationsignal pulses are applied to the ultrasonic transmitter of theultrasonic sensor unit.

An example ultrasonic sensor system may include an ultrasonictransmitter, an ultrasonic receiver, and a control unit. The ultrasonictransmitter can be configured to receive a plurality of excitationsignal pulses, wherein a frequency of the received excitation signalpulses is selected to generate an ultrasonic standing wave signal insidean ultrasonic sensor unit of the ultrasonic sensor system and whereinthe excitation signal pulses are received for a duration to allowbuildup of energy in the ultrasonic standing wave signal over a firstthreshold level. The ultrasonic receiver can be configured to detect achange in one or more characteristics of the ultrasonic standing wavesignal associated with an interaction between the ultrasonic standingwave signal and a target object. The control unit can be configured togenerate an image of the target object based on the detected change inthe one or more characteristics of the ultrasonic standing wave signal.

In certain aspects, the duration may be based on passing of a referencetime period or the number of the excitation signal pulses. The number ofexcitation signal pulses may include 4 pulses or more, 5 pulses or more,or 6 pulses or more. Detecting the change in the one or morecharacteristics of the ultrasonic standing wave signal may includedetecting a change in amplitude of the ultrasonic standing wave signal,a change in phase of the ultrasonic standing wave signal, or both.

In certain implementations of the ultrasonic sensor unit, at least theultrasonic transmitter, the ultrasonic receiver, and a thin filmtransistor (TFT) substrate form an acoustic cavity inside the ultrasonicsensor unit. The TFT substrate may be positioned between the transmitterand the receiver. In addition, in certain implementations, theultrasonic sensor unit may include a cover layer positioned over theultrasonic receiver. In certain implementations, the selected frequencyfor the excitation signal pulses is based on a thickness of an acousticcavity, a density of the acoustic cavity, a speed of sound in theacoustic cavity, or any combination thereof.

In certain implementations of the ultrasonic sensor unit, the change inthe one or more characteristics of the ultrasonic standing wave signalmay be detected while the excitation signal pulses are being applied tothe ultrasonic transmitter of the ultrasonic sensor unit. In anotherimplementation of the ultrasonic sensor unit, the change in the one ormore characteristics of the ultrasonic standing wave signal may bedetected after the excitation signal pulses are applied to theultrasonic transmitter of the ultrasonic sensor unit.

In one implementation, the ultrasonic sensor unit is an ultrasonicfingerprint sensor and the target object is a finger. An image of thefingerprint may be generated by detecting the change in the one or morecharacteristics of the ultrasonic standing wave signal by touching of aridge of the finger to a sensor surface of the ultrasonic sensor unit.

An example ultrasonic sensor system may include means for applying aplurality of excitation signal pulses to an ultrasonic transmitter of anultrasonic sensor unit, wherein a frequency of the plurality ofexcitation signal pulses is selected to generate an ultrasonic standingwave signal inside the ultrasonic sensor unit and wherein the pluralityof excitation signal pulses are applied for a duration to allow buildupof energy for the ultrasonic standing wave signal over a first thresholdlevel; means for detecting a change in one or more characteristics ofthe ultrasonic standing wave signal associated with an interactionbetween the ultrasonic standing wave signal and the target object usingan ultrasonic receiver of the ultrasonic sensor unit; and means forgenerating the image of the target object based on the detected changein the one or more characteristics of the ultrasonic standing wavesignal.

In certain aspects, the duration may be based on passing of a referencetime period or the number of the excitation signal pulses. The number ofexcitation signal pulses may include 4 pulses or more, 5 pulses or more,or 6 pulses or more. Detecting the change in the one or morecharacteristics of the ultrasonic standing wave signal may include meansfor detecting a change in amplitude of the ultrasonic standing wavesignal, a change in phase of the ultrasonic standing wave signal, orboth.

An example non-transitory computer-readable storage medium, wherein thenon-transitory computer-readable storage medium may include instructionsexecutable by a processor may include the instructions to apply aplurality of excitation signal pulses to an ultrasonic transmitter of anultrasonic sensor unit, wherein a frequency of the plurality ofexcitation signal pulses is selected to generate an ultrasonic standingwave signal inside the ultrasonic sensor unit and wherein the pluralityof excitation signal pulses are applied for a duration to allow buildupof energy for the ultrasonic standing wave signal over a first thresholdlevel; detect a change in one or more characteristics of the ultrasonicstanding wave signal associated with an interaction between theultrasonic standing wave signal and the target object using anultrasonic receiver of the ultrasonic sensor unit; and generate theimage of the target object based on the detected change in the one ormore characteristics of the ultrasonic standing wave signal.

In one implementation of the non-transitory computer-readable storagemedium the duration may be based on passing of a reference time periodor the number of the excitation signal pulses. The number of excitationsignal pulses may include 4 pulses or more, 5 pulses or more, or 6pulses or more. Detecting the change in the one or more characteristicsof the ultrasonic standing wave signal may include detecting a change inamplitude and/or phase of the ultrasonic standing wave signal. Incertain aspects, the change in the one or more characteristics of theultrasonic standing wave signal is detected while the excitation signalpulses are being applied to the ultrasonic transmitter of the ultrasonicsensor unit.

The foregoing has outlined rather broadly features and technicaladvantages of examples in order that the detailed description thatfollows can be better understood. Additional features and advantageswill be described hereinafter. The conception and specific examplesdisclosed may be readily utilized as a basis for modifying or designingother structures for carrying out the same purposes of the presentdisclosure. Such equivalent constructions do not depart from the spiritand scope of the appended claims. Features which are believed to becharacteristic of the concepts disclosed herein, both as to theirorganization and method of operation, together with associatedadvantages, will be better understood from the following descriptionwhen considered in connection with the accompanying figures. Each of thefigures is provided for the purpose of illustration and description onlyand not as a definition of the limits of the claims.

DETAILED DESCRIPTION

Several illustrative embodiments will now be described with respect tothe accompanying drawings, which form a part hereof. While particularembodiments in which one or more aspects of the disclosure may beimplemented are described below, other embodiments may be used andvarious modifications may be made without departing from the scope ofthe disclosure or the spirit of the appended claims.

Generally, as ultrasonic signals travel through a medium (e.g.,traveling waves), they may be observed as waves with crests followed bytroughs over a period of time. However, when the signals are incident onan acoustically mismatched boundary, the signals may be partiallytransmitted into the adjacent medium and partially reflected backwards.If the ultrasonic signals are traveling through a substantially solidmedium and the adjacent medium is air, most of the signal may bereflected back into the solid medium, since air tends to form a highlycompliant boundary condition and little acoustic energy may betransmitted into the air due to the high level of acoustic mismatch.

The reflected portion of the ultrasonic signal may interfere with eachconsecutively generated ultrasonic wave signal in a given medium (orplurality of mediums) within a sensor stack and produce an enhanced wavethat may amplify over time, by the plurality of signals constructivelyinterfering with each other over time. As described herein, anexcitation signal pulse may refer to an electrical signal applied to anultrasonic transmitter for generating ultrasonic wave signals within anultrasonic sensor. Each excitation signal pulse may correspond to anultrasonic wave signal generated within the sensor. The ultrasonic wavesignals may have an associated wavelength and frequency within eachmedium or layer of the sensor stack and may be generated by applyingtransmitter excitation signal pulses to one or more transmitterelectrodes using, for example, an external power source or a transmitterexcitation signal pulse generator circuit, also known as a tone-burstgenerator. One or more electrical excitation signal pulses may beapplied in succession to the ultrasonic transmitter. The frequency ofthe applied excitation signal pulses may result in a buildup of theenergy and amplitude of an ultrasonic standing wave developed, formed orotherwise generated within the ultrasonic sensor. The frequency of theapplied excitation signal pulses may be inversely related to the timeinterval between the start of a first excitation signal pulse (or cycle)and a corresponding portion of a second excitation signal pulse (orcycle). In some implementations, the frequency of the applied excitationsignal pulses may be inversely related to the time period (e.g., totaltime duration in seconds) for each pulse. In some embodiments, theexcitation signal pulse may be referred to as an electrical excitationsignal pulse, tone burst, cycle or merely signal, which may be usedinterchangeably in this disclosure without deviating from the scope ofthe invention.

With proper selection of the excitation frequency and waveform for agiven set of materials and thicknesses, the incident signal (e.g.,generated ultrasonic wave signal) and reflected signal may combine insuch a manner to constructively overlap each other as they bouncebetween the boundaries of the medium causing the ultrasonic wave toappear standing, which may be referred to as a standing wave, standingwave signal or ultrasonic standing wave signal. Furthermore, withcontinued generation and application of the excitation signal pulses,the constructive incident and reflected signals may continue to add upin amplitude as an equilibrium value is approached. The ultrasonic wavesignals in the medium may continue to increase in amplitude until theexcitation signal pulses are decreased in amplitude or are removedaltogether (e.g., stopped or no longer applied).

Proper selection of the material, thickness, and density of the variousmediums or layers in the sensor stack may result in the formation of anacoustic cavity that exhibits resonance or resonant behavior for formingthe standing wave signal at a particular frequency. An acoustic cavitymay also be referred to as an acoustic resonant cavity, a resonantacoustic cavity, a resonant cavity, an acoustic resonator or a cavityresonator, interchangeably with each other without deviating from thescope of the invention.

FIG. 1 shows a buildup of a standing wave signal, as described above,according to one or more aspects of the disclosure. FIG. 1 illustratesthe gradual buildup of the amplitude and energy of the standing wavesignal over multiple applied excitation signal pulses in the acousticcavity. The dashed line in FIG. 1 indicates the envelope correspondingto the amplitude of the standing wave signal as it builds up over time.In some implementations, the acoustic cavity is formed by various layerswithin the sensor stack and the standing wave signal is generated withthe application of the proper excitation frequency using the ultrasonictransmitter. In FIG. 1, the amplitude (e.g., displacement) and energy ofthe ultrasonic wave in the acoustic cavity increases over time as thenumber of excitation signal pulses applied to the transmitter and thenumber of generated ultrasonic wave signals increases (six pulsesshown), which may continue until an equilibrium amplitude is reached.FIG. 2 illustrates a cross-sectional view of an example configurationfor an ultrasonic sensor 200. FIG. 2 illustrates an ultrasonic sensorthat may be configured to generate a standing wave signal as describedabove. The ultrasonic sensor may have an ultrasonic transmitter (Tx)204, an ultrasonic receiver (Rx) 206, and a thin-film transistor (TFT)layer 210 including a TFT substrate and TFT pixel circuits positionedbetween the transmitter 204 and receiver 206. Although only three layersare shown in FIG. 2 for illustration purposes, other layers may also beimplemented without departing from the scope of the invention. FIG. 2 isfurther characterized with having minimal or no separation betweenportions of a finger 208 and the receiver 206. In some implementations,the top surface may be coated with a protective film, such as parylene,a urethane coating, an acrylic coating, a hard coat such as adiamond-like coating (DLC), or other suitable coating. In FIG. 2, anacoustic cavity is formed by the transmitter 204, the TFT layer 210 andthe receiver 206. FIG. 2 shows a representative waveform for a standingwave signal 212 generated in the ultrasonic sensor 200 (one and one-halfwavelengths of the standing wave signal are shown). As described infurther detail below with reference to FIG. 5, the touching of an objectsuch as a finger 208 on a sensor surface of the sensor stack may changecharacteristics of the standing wave signal such as amplitude or phaseat the receiver 206, allowing for detection of the ridges and valleys ofthe fingerprint and acquisition of an image such as a fingerprint orother biometric information with a TFT sensor array having sufficientresolution. Note that an acoustic resonant cavity may have more than oneresonant frequency, and operation at a low or fundamental resonantfrequency may be less desirable than operating at or near a higherresonant frequency. For example, enhancements to the image resolutionand quality may be obtained when operating at a higher frequency, due inpart to the smaller acoustic wavelength in the sensor unit with higheroperating frequencies.

In some implementations, the transmitter (Tx) 204 may include anelectrically conductive layer of silver and polyurethane (Ag-Ur) on alayer of piezoelectric material such as polyvinylidene flouride (PVDF)and a second layer of silver-urethane (Ag-Ur) on an opposing surface ofthe PVDF layer, with example thicknesses of 9 um, 28 um and 9 um,respectively. The TFT substrate of the TFT layer 210 may be implementedusing glass or plastic and may have a thickness of about 500 um. TFTpixel circuits may be formed on the TFT substrate using, for example, alow-temperature polysilicon, amorphous silicon, or other insulated-gatethin-film transistor process. The receiver (Tx) 206 may include apiezoelectric layer of PVDF covered with a silver-urethane layer havinga thickness of about 28 um and 9 um, respectively. The total thicknessof the acoustic cavity may be the sum total of the thickness for each ofthe layers of the ultrasonic sensor unit.

FIG. 3 illustrates a flow diagram for performing a method according toone or more aspects of the disclosure. According to one or more aspects,any and/or all of the methods and/or method steps described in the flowdiagram 300 illustrated in FIG. 3 may be implemented by electronic,mechanical and/or chemical components of a sensor either implemented asa stand-alone sensor or coupled to a computing device, such as a mobiledevice. Components of the computing device are described in greaterdetail in FIG. 14, for instance. In some implementations, one or more ofthe method steps described below with respect to FIG. 3 may beimplemented by a processor or application specific integrated circuit(ASIC) of the mobile device, such as the processor 1410 or anotherprocessor or circuitry directly coupled to the sensor. Additionally oralternatively, any and/or all of the methods and/or method stepsdescribed herein may be implemented in computer-readable instructions,such as computer-readable instructions stored on a computer-readablemedium such as the memory 1435, storage device 1425 or another computerreadable medium.

At block 302, components of the ultrasonic sensor unit, such as theultrasonic transmitter and control electronics, may be provided andconfigured to generate one or more electrical excitation signal pulsesand corresponding ultrasonic wave signals in the sensor unit. In someimplementations, the transmitter may be a piezoelectric transmitter.Piezoelectric transmitters may excite, generate or otherwise transmitmechanical motions and displacements upon application of an appropriatevoltage difference across the transmitter electrodes of thepiezoelectric transmitter.

One or more electrical excitation signal pulses may be applied to one ormore electrodes of the ultrasonic transmitter. The frequency of theexcitation signal pulses may be selected to generate an ultrasonicstanding wave signal inside the ultrasonic sensor unit. The standingwave signal may result from the constructive interference of generatedultrasonic wave signals with one or more reflected ultrasonic signalsinside an acoustic cavity of the ultrasonic sensor unit at the selectedfrequency. The reflected signals may be generated from the reflection ofone or more ultrasonic wave signals off the boundaries or interfacesbetween various mediums or layers of the ultrasonic sensor unit. Duringmanufacturing or a provisioning phase, the transmitter may be configuredto receive an appropriate number and frequency of excitation signalpulses to coincide or closely coincide with a resonant frequency of theultrasonic sensor unit, allowing for the generation of the standing wavesignal in the acoustic cavity within the ultrasonic sensor unit.

The ultrasonic sensor unit may continue to generate excitation signalpulses and ultrasonic wave signals from components of the ultrasonicsensor unit, such as the transmitter, for a reference duration. At block304, components of the ultrasonic sensor unit or components coupled tothe ultrasonic sensor unit may determine if the reference duration hascompleted for sufficient building up of the standing wave signal. Insome implementations, the reference duration may be determined bycounting the number of excitation signal pulses that have been applied.

In some implementations, the reference duration may be based on thepassing of a predetermined amount of time or reference time period. Insome implementations, the predetermined duration or reference timeperiod may be based on the number and frequency of excitation signalpulses for generating the ultrasonic wave signals. In someimplementations, the reference time period may equal the number ofexcitation signal pulses times the duration of each pulse (e.g., thenumber of cycles times the duration or period of each cycle). Asdiscussed in FIGS. 6A, 6B and 6C, the optimal number of excitationsignal pulses for some implementations may be 4 pulses or more, 5 pulsesor more, or 6 pulses or more. In some implementations, after a referencenumber of excitation signal pulses have been applied, the standing wavesignal may reach close to a saturation amplitude, wherein any additionalexcitation signal pulses do not result in a proportional energy buildupin the amplitude of the standing wave signal for the ultrasonic sensorunit and/or improvement in the resolution of the resultant image fromdetecting changes in the characteristics of the standing wave signal.

Momentarily referring to FIG. 4A and FIG. 4B, the duration of theexcitation or the number of applied excitation signal pulses to theultrasonic sensor, at least initially, may result in a standing wavesignal with increasing energy and amplitude. FIG. 4A and FIG. 4B,respectively, illustrate two graphs representing the formation of astanding wave signal with varying amplitude in an acoustic cavity of anultrasonic sensor unit. FIG. 4A illustrates two tone bursts or cycles oftransmitter excitation signal pulses applied to the terminals of anultrasonic transmitter (Tx), whereas FIG. 4B illustrates four tonebursts or cycles of applied transmitter excitation signal pulses. Eachcycle of the transmitter excitation signal may generate or add to agenerated ultrasonic wave within the acoustic cavity of the ultrasonicsensor. In FIG. 4A, the transmitter receives fewer (two) excitationsignal pulses than in FIG. 4B over a shorter period of time, resultingin a sampled waveform at the ultrasonic receiver with a lower amplitude(Lower Output in FIG. 4A) than in FIG. 4B (Higher Output in FIG. 4B)with more (four) excitation signal pulses. Buildup of a time-varyingamplitude of a generated ultrasonic wave signal 412 a within theultrasonic sensor is shown in FIG. 4A based on only two excitationsignal pulses applied to the transmitter. The generated standing wavesignal (not fully formed) may be sampled by the receiver during thesample period as shown. In comparison, the time-varying amplitude of agenerated ultrasonic wave signal 412 b resulting from four excitationsignal pulses applied to the Tx as shown in FIG. 4B has a higheramplitude than the ultrasonic wave signal 412 a, resulting in a highersensor output voltage. As illustrated, each of the electrical excitationsignal pulses applied to the transmitter may be sinusoidal in form.Alternatively, the excitation signal pulses may have other waveformssuch as square waves, short high-amplitude pulses, partial-cycle orhalf-cycle waves, or other suitable waveforms having an appropriatenumber and period for generating a standing wave signal inside theultrasonic sensor unit. It is understood that FIGS. 4A and 4B areschematic and intended to show the increasing amplitude resulting fromthe excitation of the standing wave as described elsewhere herein. Thegenerated ultrasonic wave signal 412 a is not drawn to any scale ineither amplitude or phase.

Referring back to FIG. 3, at block 306, as described previously, theultrasonic sensor unit may have an ultrasonic standing wave signalgenerated in the acoustic cavity formed by materials and layers of theultrasonic sensor unit. The standing wave signal may result from theapplication of one or more electrical excitation signal pulses to theultrasonic transmitter at the selected frequency, the generation ofultrasonic wave signals within the sensor stack, and the constructiveinterference of the generated ultrasonic wave signals with one or morereflected ultrasonic signals inside an acoustic cavity of the sensorunit.

At block 308, components of the ultrasonic sensor unit or a computingdevice coupled to the ultrasonic sensor unit, such as the receiver, maydetect a change in one or more characteristics of the standing wavesignal associated with an interaction between the standing wave signaland a target object. The changes in the characteristics of the standingwave signal may include the amplitude and/or phase of the standing wavesignal as measured at the receiver. In some implementations, theamplitude of the standing wave signal may be detected by acquiring andmeasuring the peak signal voltage generated across the piezoelectricreceiver layer with the pixel circuits in the TFT sensor array. In someimplementations, the phase of the standing wave signal may be detectedby acquiring and measuring the voltage generated across thepiezoelectric receiver layer with a relatively narrow sample window(e.g., sampling period) at a prescribed time after the start or stop ofthe excitation signal pulses.

Again, referring to FIGS. 4A and 4B, in some implementations, thetransmitting period and the receiving (or detecting) period may occurduring different time intervals or with different time delays.Embodiments described herein enable components, such as the receiver, tosample the standing wave signal after the generation and application ofthe excitation signal pulses. In some implementations, the receiver maysample the standing wave signal immediately after the application of theexcitation signal pulses, resulting in a possible reduction inelectrical interference between the applied signal pulses and thedetected sensor output signals. In some implementations, the receivermay sample the standing wave signal a prescribed time delay after theapplication of the excitation signal pulses to achieve, for example,higher image contrast or higher image quality. In some implementations,the receiver may sample the standing wave signal during the applicationof the excitation signal pulses, allowing the ultrasonic sensor unit tobe more responsive. In some embodiments, the ultrasonic sensor unitdescribed herein may detect a change in the standing wave signal that isaccumulating (constructively) energy and amplitude rather than detectinga reflection that may result in destructive interference with thegenerated ultrasonic wave signals. With respect to FIG. 4A and FIG. 4B,signals may be sampled during a sample window of a sample mode, forexample, with a peak detector in each of the sensor pixel circuits ofthe sensor array. During a hold mode, the sampled signal may be held forsubsequent clocking out of the sensor image information. During a blockmode, the pixel circuits may be prevented (blocked) from acquiring asignal. In some implementations, a corresponding sample, hold or blockvoltage level may be applied to the receiver bias electrode (Rx) toenter these various modes. In some implementations, the duration of thesample window may be referred to as a range gate window, and the timedelay between the start of the excitation signal pulses and the openingof the sample window may be referred to as a range gate delay. Bycontrolling the width and timing of the sample window, the amplitude andphase of the standing wave signal may be detected.

Referring back to FIG. 3, due to the constructive buildup of thestanding wave signal, optionally, at block 310, components of theultrasonic sensor unit or a computing device coupled to the ultrasonicsensor unit may continue to generate and apply excitation signal pulsesto the transmitter, while the receiver may receive and detect changesassociated with the standing wave signal. Some implementations allow thetransmitter to continue to transmit and build up the standing wavesignal while the receiver detects the change in one or morecharacteristics of the standing wave signal such as the amplitude and/orphase of the standing wave signal. This may allow the ultrasonic sensorto continuously receive and/or detect changes in characteristics of thestanding wave signal without switching the transmitter and receiver onand off, increasing the responsiveness of the ultrasonic sensor unit.Alternatively, the receiver may sample the standing wave signal afterthe buildup of the ultrasonic standing wave has occurred and applicationof the excitation signal pulses has been stopped. In someimplementations, the receiver may sample the standing wave signalimmediately after or a prescribed time delay after the application ofthe excitation signal pulses.

At block 312, components of the ultrasonic sensor unit or a computingdevice coupled to the ultrasonic sensor unit may acquire sensor outputsignals from sensor pixel circuitry of the ultrasonic receiver andgenerate an image of a target object based on detecting the change inone or more characteristics of the standing wave signal. The targetobject, such as a finger, may be positioned on a sensor surface of thesensor unit. Once the receiver samples the standing wave signal, thesensor unit may acquire the samples and convert the samples from analogto digital sensor image information. The sensor image information may befurther processed on an ASIC or a processor to discern the ultrasonicimage acquired by the sensor, such as fingerprint images of a fingerplaced on the sensor unit. In some implementations, generating an imagemay incur additional manipulations to the sensor image information suchas contrast enhancement, gray-scale adjustments, sizing and formattingto allow displaying of the generated imaging in a suitable manner. Insome implementations, generating an image may involve minimal signalprocessing, and generating an image may include only placing or storingthe detected changes in memory on a pixel-by-pixel basis to allow forfurther processing, such as user verification, authorization, oridentification.

It should be appreciated that the specific steps illustrated in FIG. 3provide a particular method of switching between modes of operation,according to various aspects of the disclosure. Other sequences of stepsmay also be performed accordingly in alternative embodiments. Forexample, alternative embodiments of the present disclosure may performthe steps outlined above in a different order. Moreover, the individualsteps illustrated in FIG. 3 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize and appreciate many variations, modifications, andalternatives of the process.

FIG. 5 illustrates a cross-sectional view of an example ultrasonicsensor unit. As shown in FIG. 5, the ultrasonic sensor unit 502 in oneconfiguration may have an ultrasonic transmitter (Tx) 504, a receiver(Rx) 506, a TFT substrate 508 positioned between the transmitter 504 andthe receiver 506, and a platen or display/cover glass 510 disposed overthe receiver. Adhesive attachment layers and one or more coating layersmay be included (not shown for clarity). For applying the excitationsignal pulses and generating the ultrasonic wave signals, thetransmitter 504 may have one or more electrodes on each side of thepiezoelectric transmitter layer to apply the potential difference fordriving the transmitter. Similarly, the receiver 506 may have a receiverbias (Rx Bias) electrode on one side of the piezoelectric receiver layerand a plurality of pixel input electrodes (here Rx Electrodes) coupledto the pixel circuitry of the TFT substrate 508. The pixel circuitry maybe configured to detect a change in one or more characteristics of thestanding wave signal in the acoustic cavity of the ultrasonic sensorunit. FIG. 13 describes aspects of the pixel circuitry in more detail.

FIG. 5 also illustrates changes in one or more characteristics of theultrasonic standing wave signal associated with the interaction betweenthe standing wave signal in the acoustic cavity of the ultrasonic sensorunit 502 and the target object. Detecting the change in the one or morecharacteristics of the signal may include detecting the change in theamplitude and/or phase of the signal. FIG. 5 illustrates an ultrasonicfingerprint sensor and the target object may be a finger. In FIG. 5, thedifference in the acoustic energy reflected by air and the acousticenergy reflected by the finger may result in differences in thecharacteristics of the standing wave signal. For example, an air gap,formed by the fingerprint valley 518 as shown in FIG. 5, may onlyminimally change the amplitude and phase of the standing wave signal514. On the other hand, touching of a fingerprint ridge 520 against anexposed portion of the sensor unit (e.g., sensor surface) may dampen theenergy of the standing wave signal and may cause a shift in thefrequency, amplitude and/or phase of the standing wave signal 512, asshown in FIG. 5.

FIG. 6A illustrates a graph showing the frequency response of anexemplary sensor output, based on transmitter excitation signal pulsesreferred to here as tone bursts (TB) or cycles in the figures. The‘TB_(on)-TB_(off)’, shown as the y-axis in FIG. 6A, indicates thestrength of the standing signal wave, shown in millivolts, in responseto the various excitation frequencies, shown as the x-axis. The sensoroutput voltage ‘TB_(on)-TB_(off)’ indicates a difference between thesensor output voltage with the application of the tone bursts (TB_(on))and the sensor output voltage without the application of tone bursts(TB_(off)). The subtraction tends to remove or mitigate any variationsin quiescent or background output signals within the sensor array. FIG.6A further illustrates eight different waveforms, each corresponding tothe frequency response for a certain number of cycles indicated in thelegend that is displayed in the top right corner of FIG. 6A. Accordingto the graph, the frequency response of the ultrasonic sensor unit islargest at around 11 MHz. Furthermore, as shown in FIG. 6A, as thenumber of cycles increase, the response at around 11 MHz increasessignificantly with the application of between about four and six cyclesand then tends to level off with further increases in the number ofapplied cycles (around 6-8 cycles).

FIG. 6B illustrates the increase in the response and then a leveling offof the resonance amplitude with respect to the number of applied cycles.Similar to FIG. 6A, the y-axis represents ‘TB_(on)-TB_(off)’. The x-axisrepresents the number of cycles. In FIG. 6B, the resonance is built upas the number of cycle increases. The change in the resonance amplitudeis visible between about four and six cycles, after which the resonanceresponse levels off gradually.

FIG. 6C visually shows successive improvement in the image quality of afingerprint image as the number of excitation signal pulses or cycles isincreased and saturation of the acquired data is approached, for theimplementation shown with reference to FIGS. 6A and 6B. In other words,as the number of input cycles increase, more energy accumulates insidethe resonant cavity as indicated by the increase in signal strength. Forthis implementation, the most efficient transduction and image qualitymay be found at about 6 cycles and 11 MHz. A fingerprint image begins tobe revealed after about four cycles. In at least this example, theoutput signal and image quality tends to saturate between about six andeight cycles. A threshold level may be determined that allows sufficientbuildup of energy and/or amplitude of the standing wave signal. Thethreshold level may be based on the desired signal strength or clarityof the generated image. In some implementations, the threshold level maybe a threshold number of applied excitation cycles to achievesatisfactory sensor output signal levels, image quality, or imagecontrast. In some implementations, the threshold level may be a minimumnumber of millivolts in the sensor output signal (e.g.,TB_(on)-TB_(off)) to generate an image.

FIG. 7A shows an example top view of an implementation of an ultrasonicfingerprint sensor. FIG. 7A shows the leads connecting to the ultrasonicreceiver (Rx) and the ultrasonic transmitter (Tx). FIG. 7B shows anexample side view of the ultrasonic fingerprint sensor array. In theexample implementation shown, the thickness of the sensor is about 500um and the size of the sensor is about 1″×1″. However, aspects of thedisclosure are in no way limited by the thickness or the size shown inthe illustrative figures. For example, in other implementations, theultrasonic sensor may have a TFT sensor array with sensor pixels havingabout a 50 um pixel pitch, about 500 pixels per inch, and active sensorareas of 15 mm×6 mm to full display sizes, 11 mm×11 mm to 1″×1″, andother sizes. Furthermore, the ultrasonic sensor may have a low profile(˜1 mm or less), operating at a high operational frequency (about 5-25MHz).

FIG. 8 illustrates a cross-sectional view of another exampleconfiguration for an ultrasonic sensor unit. FIG. 8 illustrates anultrasonic sensor unit 800 that may have an ultrasonic transmitter (Tx)802 at the bottom, a receiver (Rx) 806, a TFT layer 804 between thetransmitter 802 and the receiver 806, and a cover layer 808 (i.e. glassor plastic), that may serve as a cover glass, cover lens or platen.Although only four layers are shown in FIG. 8 for illustration purposes,other layers (such as those illustrated with respect to FIG. 11A) mayalso be implemented without departing from the scope of the invention. Aprotective coating (not shown) may be included on the surface of thecover layer 808 to provide environmental protection and may also serveas an impedance matching layer. As described with reference to FIG. 5,the touching of an object, such as a finger 810 on the sensor surface,may change the characteristics of the standing wave signal 812, allowingfor detection of the ridges and valleys of the fingerprint. Aspects ofthe disclosure allow for optimal image capture for ultrasonic sensorswith various cover materials and cover layers of various thicknesses.For instance, the same ultrasonic sensor unit depicted in FIG. 2 may beused as the sensor in FIG. 8, with an added cover layer 808 andappropriate modifications of the excitation frequency, since theacoustic cavity may include the cover layer 808. In someimplementations, the frequency of transmission from the transmitter 802may be configured or adjusted at manufacturing or during a provisioningphase to determine an optimal resonance frequency and to operate at thatfrequency for improved image acquisition. Configuring or/and adjustingthe frequency may account for the additional thickness added by thecover layer and/or coatings on the ultrasonic sensor (two and one-halfwavelengths of the standing wave signal are shown in FIG. 8).

In some implementations, the transmitter (Tx) 802 may be formed using asilver-urethane (Ag-Ur) layer, a PVDF layer, and a second layer ofAg-Ur, with example thicknesses of 9 um, 28 um, and 9 um, respectively.The TFT layer 804 may be implemented using glass or plastic substratesand may be about 500 um in thickness. The receiver (Tx) 806 may beimplemented using a layer of Ag-Ur on PVDF, having a thickness of about9 um and 28 um, respectively. The cover layer 808, which may serve as aplaten for a fingerprint sensor or as a cover glass for a display, maybe a variety of different thicknesses, such as 127 um, 254 um or othersuitable thickness. In addition, the ultrasonic sensor unit 800 may havea protective cover or coating that is scratch and abrasion resistantwith a thickness anywhere from less than about 10 um to about 50 um ormore. The cover layer 808 may have a coating disposed thereon thatserves as an impedance matching layer between the cover layer 808 and atarget object such as a finger 810. The total thickness for the acousticcavity may be the sum total of the thickness for each of the layers ofthe ultrasonic sensor unit.

FIG. 9 shows an example ultrasonic sensor with a cover layer above theTFT substrate and receiver. In FIG. 9, the cover layer is highlighted bybox 902. As discussed previously, embodiments may be adapted to accountfor the additional cover layer or surface coatings by adjusting thetransmitter excitation frequency. Polycarbonate is shown and mentionedas only one example material that may be used as a cover layer. Othermaterials may be used for the cover layer such as plastic, ceramic,sapphire, composite materials, metal and metal alloys, metal-filledpolymers, or glass, without departing from the invention.

As described above, embodiments of the disclosure allow for differentthickness and materials for the cover layers and coatings, by adjustingthe resonance frequency for the acoustic cavity. This flexibility allowsthe device manufacturers to provision the frequency for the ultrasonicsensor unit integrated into their systems based on various cover layersor coatings used by the manufacturers.

FIG. 10A and FIG. 10B illustrate graphs showing the frequency responseof an example ultrasonic sensor, with different numbers of excitationsignal pulses (or cycles) for two different thicknesses of polycarbonatecover layers. In FIG. 10A, the ultrasonic sensor is implemented with a127-um thick polycarbonate cover. FIG. 10A shows a graph that has anoptimal frequency for image acquisition at or around a peak frequency ofabout 10 MHz and a fingerprint image taken with six excitation cycles at10 MHz.

FIG. 10B has a different cover thickness than FIG. 10A. In FIG. 10B, theultrasonic sensor is implemented with a 254-um thick polycarbonate coverlayer. FIG. 10B shows a graph that has an optimal frequency for imageacquisition at or around a peak frequency of about 9 MHz and afingerprint image taken with six excitation cycles at 9 MHz. FIG. 10Cillustrates differences in the sensor output voltages at the resonantfrequencies for each sensor with respect to the different number ofexcitation signal pulses or cycles. FIGS. 10A, 10B and 10C illustratethat techniques described herein allow for cover layers and coatingswith varying thicknesses over the ultrasonic sensor unit.

FIG. 11A illustrates an exploded view displaying various layers of amaterial stack for an example ultrasonic sensor unit with an acousticcavity resonator. FIG. 11A shows, top to bottom, a cover layer, areceiver layer, a TFT sensor array, and a transmitter layer. The coverlayer may be a cover glass or coating (e.g., glass, polycarbonate,acrylic, parylene or any other appropriate material serving as a coveror coating). The receiver layer may have a receiver bias electrode(e.g., silver-urethane, nickel/copper (Ni/Cu), or indium tin oxide(ITO)) disposed on a piezoelectric receiver layer (e.g., polyvinylidenefluoride (PVDF) or polyvinylidene fluoride-trifluoroethylene copolymer(PVDF-TrFE)). The TFT sensor array may have pixel circuitry formed on aTFT substrate (e.g., glass or plastic), similar to circuitry shown inFIG. 13. The transmitter layer may have a transmitter electrode (e.g.,silver-urethane or Ni/Cu) disposed on a piezoelectric transmitter layer(e.g., PVDF or PVDF-TrFE), and another transmitter electrode disposed onthe opposing side of the piezoelectric transmitter layer. Variousadhesive layers and optional coating layers have been omitted from theillustration of FIG. 11A for clarity. Furthermore, in addition to thelayers shown in FIG. 11A, other layers may also be implemented withoutdeparting from the scope of the invention.

FIG. 11B illustrates an assembled view of an example ultrasonic sensorunit with an acoustic cavity resonator. FIG. 11B shows an assembled viewof the cover layer, the receiver layer, the TFT sensor array and thetransmitter layer. In other configurations (not shown), an acousticcavity resonator may be formed by stacking both the transmitter andreceiver layers along with associated electrodes and adhesives betweenthe TFT substrate and the cover layer. The transmitter may include oneor more piezoelectric layers and electrodes to allow single-ended ordoubled-ended drive schemes and self-shielding by grounding one or moreof the transmitter electrodes. In other configurations (not shown), anacoustic cavity may be formed by placing stacked transmitter andreceiver layers along with associated electrodes, adhesive layers, andcoatings on top of the TFT substrate, using the TFT substrate as asemi-rigid boundary condition on one side of the stack and air on theother side that serves as a free boundary condition. In otherconfigurations (not shown), a single-layer transmitter and receiver maybe placed between the TFT substrate and a cover layer to form theacoustic resonant cavity. In other configurations (not shown), asingle-layer transmitter and receiver may be placed between the TFTsubstrate and air to form the acoustic resonant cavity.

In at least one embodiment, a few of the key parameters for the acousticcavity may include the thickness of and speed of sound within thevarious layers that form the acoustic cavity. The speed of sound, inturn, depends in part on the mass density and elastic moduli of theincorporated materials. The thickness of the acoustic cavity may dependor at least correlate to the thickness of the TFT substrate,piezoelectric layers, electrodes and adhesives (e.g., epoxy or pressuresensitive adhesive (PSA)), and any cover layers, backing layers, orcoatings. The effective density of the acoustic cavity may depend or atleast correlate to the density of the substrate material, piezoelectricmaterials, electrode and adhesive materials, and any cover or coatingmaterials. Similarly, the effective speed of sound in the acousticcavity may depend on the choice of substrates, piezoelectric materials,electrode materials, adhesives, and any cover or coating materials.

FIGS. 12A, 12B, 12C and 12D illustrate various example configurationsand placements of an example ultrasonic sensor unit with a display orcover glass of a mobile device. In some implementations, the sensors maybe positioned on a bezel, on the side, or on the back of a mobile deviceenclosure, such as the mobile device shown in FIG. 14. In someinstances, the placement of the ultrasonic sensor unit may determinecharacteristics such as the effective thickness, density and speed ofsound of the acoustic cavity. In FIG. 12A, the ultrasonic sensor unit isshown at the periphery of the enclosure with the display cover glass ontop of the sensor. In FIG. 12B, the ultrasonic sensor unit is shownbelow the display cover glass, the display color filter glass and thedisplay TFT substrate. In FIG. 12C, the ultrasonic sensor unit isintegrated into the TFT substrate allowing for the whole screen or largeportion of the screen to have ultrasonic sensing capability. In FIG.12D, the ultrasonic sensor unit may be positioned as a stand-alonesensor outside of the display region, or as part of a button (mechanicalor non-mechanical). In such cases, the enclosure of the button mayadditionally determine the characteristics for the acoustic cavity. Insome embodiments, for an ultrasonic fingerprint sensor, the sensitivityof the sensor and density of the pixels (e.g., resolution) may beselected based on if the sensor is used for authenticating ornon-authenticating purposes. Authenticating purposes, such as unlockinga mobile device or accessing an account using the fingerprint sensor,may need a higher resolution that clearly differentiates the ridges andvalleys on the finger. Non-authenticating uses of the ultrasonic sensorunit may include simply pressing of a button or detection of the fingertouching the surface and may allow acquisition of lower resolutionimages. In some implementations, the fingerprint sensor may be placedagainst and coupled to a metal or plastic cover of a mobile deviceenclosure.

FIG. 13 illustrates a block diagram for an example representation of anultrasonic sensor unit. An example of an ultrasonic sensor unit is anultrasonic fingerprint sensor. The ultrasonic sensor unit may have a TFTsubstrate that has an ultrasonic transmitter 1304 and a receiver coupledto an ultrasonic pixel circuit array 1302. The ultrasonic pixel circuitarray 1302 and an overlying piezoelectric receiver layer, acting as thereceiver, may be disposed on the TFT substrate. Furthermore, FIG. 13shows the components for converting the sensor output signals fromanalog to digital using an analog to digital converter (ADC) 1306,selecting the appropriate pixel output signals (e.g., rows or columns)using one or more multiplexers 1308 and associated gate drivers, and acontrol unit 1310 and/or data processor 1312 for processing the sensorinformation. FIG. 13 also illustrates drivers for biasing and excitingthe Rx and Tx layers of the ultrasonic sensor unit. In someimplementations, the control unit 1310 and/or data processor 1312 mayuse the processor 1410 described in FIG. 14. In some implementations,the control unit 1310 and data processor 1312 may use an applicationspecific integrated circuit (ASIC) or a field programmable gate array(FPGA) for processing information. In some implementations, the controlunit 1310 and/or the data processor 1312 may be used for acquiringsensor output signals from the pixel circuitry, and forming orgenerating the image from the information obtained from the ultrasonicsensor pixel circuitry array 1302. Whereas thin-film transistors formedon glass or plastic TFT substrates have been described above, inalternative forms, a silicon substrate with transistors formed thereonor therein may be substituted without limitation for the TFT substratesthroughout this disclosure.

FIG. 14 illustrates an example computing device incorporating parts ofthe device employed in practicing embodiments of the invention. Acomputing device as illustrated in FIG. 14 may be incorporated as partof any computerized system, herein. For example, computing device 1400may represent some of the components of a mobile device or a computingdevice. Examples of a computing device 1400 include, but are not limitedto, desktops, workstations, personal computers, supercomputers, videogame consoles, tablets, smart phones, laptops, netbooks, wearable healthmonitors, or other portable devices. FIG. 14 provides a schematicillustration of one embodiment of a computing device 1400 that mayperform the methods provided by various other embodiments, as describedherein, and/or may function as the host computing device, a remotekiosk/terminal, a point-of-sale device, a mobile multifunction device, aset-top box and/or a computing device. FIG. 14 is meant only to providea generalized illustration of various components, any or all of whichmay be utilized as appropriate. FIG. 14, therefore, broadly illustrateshow individual system elements may be implemented in a relativelyseparated or relatively more integrated manner.

The computing device 1400 is shown comprising hardware elements that maybe electrically coupled via a bus 1405 (or may otherwise be incommunication, as appropriate). The hardware elements may include one ormore processors 1410, including without limitation one or moregeneral-purpose processors and/or one or more special-purpose processors(such as digital signal processing chips, graphics accelerationprocessors, and/or the like); one or more input devices 1415, which mayinclude without limitation a camera, sensor(s) 1450, a mouse, a keyboardand/or the like; and one or more output devices 1420, which may includewithout limitation a display unit, a printer and/or the like. Sensor(s)1450 may include ultrasonic sensors as described herein and/or otherimaging sensors. Specifically some devices may include ultrasonicfingerprint sensors. In some instances, the processing for theultrasonic sensor may be performed by the one or more processors 1410.In another embodiment, control logic, implemented as an ASIC, FPGA orany other suitable means, may be coupled to the ultrasonic sensor unitfor performing processing for the ultrasonic sensor unit. In someimplementations, the computing device 1400 is a mobile device and thesensor(s) 1450 includes an ultrasonic sensor unit coupled to the mobiledevice.

The computing device 1400 may further include (and/or be incommunication with) one or more non-transitory storage devices 1425,which may comprise, without limitation, local and/or network accessiblestorage, and/or may include, without limitation, a disk drive, a drivearray, an optical storage device, a solid-form storage device such as arandom access memory (“RAM”) and/or a read-only memory (“ROM”), whichmay be programmable, flash-updateable and/or the like. Such storagedevices may be configured to implement any appropriate data storage,including without limitation, various file systems, database structures,and/or the like.

The computing device 1400 might also include a communications subsystem1430. The communications subsystem 1430 may include a transceiver forreceiving and transmitting data or a wired and/or wireless medium. Thecommunications subsystem 1430 may also include without limitation amodem, a network card (wireless or wired), an infrared communicationdevice, a wireless communication device and/or chipset (such as aBluetooth™ device, an 802.11 device, a WiFi device, a WiMax device,cellular communication facilities, etc.), and/or the like. Thecommunications subsystem 1430 may permit data to be exchanged with anetwork (such as the network described below, to name one example),other computing devices, and/or any other devices described herein. Inmany embodiments, the computing device 1400 will further comprise anon-transitory working memory 1435, which may include a RAM or ROMdevice, as described above.

The computing device 1400 may comprise software elements, shown as beingcurrently located within the working memory 1435, including an operatingsystem 1440, device drivers, executable libraries, and/or other code,such as one or more application programs 1445, which may comprisecomputer programs provided by various embodiments, and/or may bedesigned to implement methods, and/or configure systems, provided byother embodiments, as described herein. Merely by way of example, one ormore procedures described with respect to the method(s) discussed abovemight be implemented as code and/or instructions executable by acomputer (and/or a processor within a computer); in an aspect, then,such code and/or instructions may be used to configure and/or adapt ageneral purpose computer (or other device) to perform one or moreoperations in accordance with the described methods.

A set of these instructions and/or code might be stored on acomputer-readable storage medium, such as the storage device(s) 1425described above. In some cases, the storage medium might be incorporatedwithin a computing device, such as computing device 1400. In otherembodiments, the storage medium might be separate from a computingdevice (e.g., a removable medium, such as a compact disc), and/orprovided in an installation package, such that the storage medium may beused to program, configure and/or adapt a general purpose computer withthe instructions/code stored thereon. These instructions might take theform of executable code, which is executable by the computing device1400 and/or might take the form of source and/or installable code,which, upon compilation and/or installation on the computing device 1400(e.g., using any of a variety of generally available compilers,installation programs, compression/decompression utilities, etc.) thentakes the form of executable code.

Substantial variations may be made in accordance with specificrequirements. For example, customized hardware might also be used,and/or particular elements might be implemented in hardware, software(including portable software, such as applets, etc.), or both. Further,connection to other computing devices 1400 such as network input/outputdevices may be employed.

Some embodiments may employ a computing device (such as the computingdevice 1400) to perform methods in accordance with the disclosure. Forexample, some or all of the procedures of the described methods may beperformed by the computing device 1400 in response to processor 1410executing one or more sequences of one or more instructions (which mightbe incorporated into the operating system 1440 and/or other code, suchas an application program 1445) contained in the working memory 1435.Such instructions may be read into the working memory 1435 from anothercomputer-readable medium, such as one or more of the storage device(s)1425. Merely by way of example, execution of the sequences ofinstructions contained in the working memory 1435 might cause theprocessor(s) 1410 to perform one or more procedures of the methodsdescribed herein.

The terms “machine-readable medium” and “computer-readable medium,” asused herein, refer to any medium that participates in providing datathat causes a machine to operate in a specific fashion. In an embodimentimplemented using the computing device 1400, various computer-readablemedia might be involved in providing instructions/code to processor(s)1410 for execution and/or might be used to store and/or carry suchinstructions/code (e.g., as signals). In many implementations, acomputer-readable medium is a physical and/or tangible storage medium.Such a medium may take many forms, including, but not limited to,non-volatile media, volatile media, and transmission media. Non-volatilemedia include, for example, optical and/or magnetic disks, such as thestorage device(s) 1425. Volatile media include, without limitation,dynamic memory, such as the working memory 1435. Transmission mediainclude, without limitation, coaxial cables, copper wire and fiberoptics, including the wires that comprise the bus 1405, as well as thevarious components of the communications subsystem 1430 (and/or themedia by which the communications subsystem 1430 provides communicationwith other devices). Hence, transmission media may also take the form ofwaves (including without limitation radio, acoustic and/or light waves,such as those generated during radio-wave and infrared datacommunications). In an alternate embodiment, event-driven components anddevices, such as cameras, may be used, where some of the processing maybe performed in analog domain.

Common forms of physical and/or tangible computer-readable mediainclude, for example, a floppy disk, a flexible disk, hard disk,magnetic tape, or any other magnetic medium, a CD-ROM, any other opticalmedium, punchcards, papertape, any other physical medium with patternsof holes, a RAM, a PROM, EPROM, a FLASH-EPROM, any other memory chip orcartridge, a carrier wave as described hereinafter, or any other mediumfrom which a computer may read instructions and/or code.

Various forms of computer-readable media may be involved in carrying oneor more sequences of one or more instructions to the processor(s) 1410for execution. Merely by way of example, the instructions may initiallybe carried on a magnetic disk and/or optical disc of a remote computer.A remote computer might load the instructions into its dynamic memoryand send the instructions as signals over a transmission medium to bereceived and/or executed by the computing device 1400. These signals,which might be in the form of electromagnetic signals, acoustic signals,optical signals and/or the like, are all examples of carrier waves onwhich instructions may be encoded, in accordance with variousembodiments of the invention.

The communications subsystem 1430 (and/or components thereof) generallywill receive the signals, and the bus 1405 then might carry the signals(and/or the data, instructions, etc. carried by the signals) to theworking memory 1435, from which the processor(s) 1410 retrieves andexecutes the instructions. The instructions received by the workingmemory 1435 may optionally be stored on a non-transitory storage device1425 either before or after execution by the processor(s) 1410.

The methods, systems, and devices discussed above are examples. Variousembodiments may omit, substitute, or add various procedures orcomponents as appropriate. For instance, in alternative configurations,the methods described may be performed in an order different from thatdescribed, and/or various stages may be added, omitted, and/or combined.Also, features described with respect to some embodiments may becombined in various other embodiments. Different aspects and elements ofthe embodiments may be combined in a similar manner. Also, technologyevolves and, thus, many of the elements are examples that do not limitthe scope of the disclosure to those specific examples.

Specific details are given in the description to provide a thoroughunderstanding of the embodiments. However, embodiments may be practicedwithout these specific details. For example, well-known circuits,processes, algorithms, structures, and techniques have been shownwithout unnecessary detail in order to avoid obscuring the embodiments.This description provides example embodiments only, and is not intendedto limit the scope, applicability, or configuration of the invention.Rather, the preceding description of the embodiments will provide thoseskilled in the art with an enabling description for implementingembodiments of the invention. Various changes may be made in thefunction and arrangement of elements without departing from the spiritand scope of the invention.

Also, some embodiments are described as processes depicted as flowdiagrams or block diagrams. Although each may describe the operations asa sequential process, many of the operations may be performed inparallel or concurrently. In addition, the order of the operations maybe rearranged. A process may have additional steps not included in thefigure. Furthermore, embodiments of the methods may be implemented byhardware, software, firmware, middleware, microcode, hardwaredescription languages, or any combination thereof. When implemented insoftware, firmware, middleware, or microcode, the program code or codesegments to perform the associated tasks may be stored in acomputer-readable medium such as a storage medium. Processors mayperform the associated tasks.

Having described several embodiments, various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the disclosure. For example, the above elements may merely bea component of a larger system, wherein other rules may take precedenceover or otherwise modify the application of the invention. Also, anumber of steps may be undertaken before, during, or after the aboveelements are considered. Accordingly, the above description does notlimit the scope of the disclosure.

What is claimed is:
 1. A method for generating an image of a targetobject, comprising: applying a plurality of excitation signal pulses toan ultrasonic transmitter of an ultrasonic sensor unit, wherein afrequency of the plurality of excitation signal pulses is selected togenerate an ultrasonic standing wave inside a cover glass layer of astack of material layers, the cover glass layer configured for contactwith the target object, and wherein the plurality of excitation signalpulses are applied for a duration to allow buildup of energy for theultrasonic standing wave over a first threshold level; detecting achange in one or more characteristics of the ultrasonic standing waveassociated with an interaction between the ultrasonic standing wave andthe target object using an ultrasonic receiver of the ultrasonic sensorunit; and generating the image of the target object based on thedetected change in the one or more characteristics of the ultrasonicstanding wave.
 2. The method of claim 1, wherein the ultrasonic standingwave results from constructive interference of generated ultrasonic wavesignals with one or more reflected ultrasonic signals inside acousticcavity.
 3. The method of claim 1, wherein the change in the one or morecharacteristics of the ultrasonic standing wave is detected while theexcitation signal pulses are being applied to the ultrasonic transmitterof the ultrasonic sensor unit.
 4. The method of claim 1, wherein thechange in the one or more characteristics of the ultrasonic standingwave is detected after the excitation signal pulses are applied to theultrasonic transmitter of the ultrasonic sensor unit.
 5. The method ofclaim 1, wherein the duration is based on passing of a reference timeperiod.
 6. The method of claim 1, wherein the duration is based on anumber of the excitation signal pulses.
 7. The method of claim 6,wherein the number of the excitation signal pulses is 4 pulses or more,5 pulses or more, or 6 pulses or more.
 8. The method of claim 1, whereindetecting the change in the one or more characteristics of theultrasonic standing wave includes detecting a change in amplitude of theultrasonic standing wave, a change in phase of the ultrasonic standingwave, or both a change in amplitude and a change in phase of theultrasonic standing wave.
 9. The method of claim 1, wherein theultrasonic sensor unit is an ultrasonic fingerprint sensor and thetarget object is a finger.
 10. The method of claim 9, wherein an imageof the fingerprint is generated by detecting the change in the one ormore characteristics of the ultrasonic standing wave by touching of aridge of the finger to a surface of the cover glass layer of theultrasonic sensor unit.
 11. The method of claim 1, wherein the pluralityof excitation signal pulses is applied using a single-ended ordouble-ended drive scheme electrically connected to one or moretransmitter excitation electrodes associated with one or morepiezoelectric layers included in the ultrasonic transmitter.
 12. Themethod of claim 1, wherein the excitation signal pulses to generate theultrasonic standing wave include at least one of a sinusoidal wave, asquare wave, a short high-amplitude pulse, a partial-cycle wave, or ahalf-cycle wave.
 13. An ultrasonic sensor system, comprising: a stack ofmaterial layers including a cover glass layer, the cover glass layerconfigured for contact with a target object; and an ultrasonictransmitter in acoustic contact with the stack of material layers, theultrasonic transmitter configured to receive a plurality of excitationsignal pulses, wherein a frequency of the received excitation signalpulses is selected to generate an ultrasonic standing wave inside thecover glass layer and wherein the excitation signal pulses are receivedfor a duration to allow buildup of energy in the ultrasonic standingwave over a first threshold level.
 14. The ultrasonic sensor system ofclaim 13, wherein the selected frequency for the excitation signalpulses is based on an effective thickness of acoustic cavity, aneffective density of the acoustic cavity, an effective speed of sound inthe acoustic cavity, or a combination thereof.
 15. The ultrasonic sensorsystem of claim 13, wherein the stack of material layers includes adisplay.
 16. The ultrasonic sensor system of claim 13, wherein the coverglass layer includes a surface coating.
 17. The ultrasonic sensor systemof claim 13, further comprising: an ultrasonic receiver configured todetect a change in one or more characteristics of the ultrasonicstanding wave associated with an interaction between the ultrasonicstanding wave and the target object; and a control unit configured togenerate an image of the target object based on the detected change inthe one or more characteristics of the ultrasonic standing wave.
 18. Theultrasonic sensor system of claim 17, wherein the ultrasonic receiverincludes an active sensor area having transistors formed on a siliconsubstrate, thin-film transistors (TFTs) formed on a glass or plasticsubstrate, or thin-film transistors integrated on a display TFTsubstrate.
 19. The ultrasonic sensor system of claim 18, wherein theactive sensor area extends across at least a portion of a display. 20.The ultrasonic sensor system of claim 13, further comprising: anultrasonic receiver configured to detect a change in one or morecharacteristics of the ultrasonic standing wave associated with aninteraction between the ultrasonic standing wave and the target object,wherein the ultrasonic transmitter and the ultrasonic receiver include asingle piezoelectric layer.
 21. The ultrasonic sensor system of claim20, wherein the piezoelectric layer includes a layer of polyvinylidenefluoride (PVDF) or polyvinylidene fluoride-trifluoroethylene copolymer(PVDF-TrFE).
 22. The ultrasonic sensor system of claim 13, wherein theultrasonic transmitter includes one or more piezoelectric layers and oneor more transmitter excitation electrodes associated with each of theone or more piezoelectric layers.
 23. The ultrasonic sensor system ofclaim 22, wherein the one or more piezoelectric layers and the one ormore transmitter excitation electrodes are configured to allowsingle-ended or double-ended drive schemes.
 24. The ultrasonic sensorsystem of claim 22, wherein the one or more piezoelectric layers and theone or more transmitter excitation electrodes are configured to allowself-shielding by grounding one or more of the transmitter excitationelectrodes.
 25. An ultrasonic sensor system, comprising: means forapplying a plurality of excitation signal pulses to an ultrasonictransmitter of an ultrasonic sensor unit, wherein a frequency of theexcitation signal pulses is selected to generate an ultrasonic standingwave inside a cover glass layer of a stack of material layers andwherein the excitation signal pulses are applied for a duration to allowbuildup of energy for the ultrasonic standing wave over a firstthreshold level; means for detecting a change in one or morecharacteristics of the ultrasonic standing wave associated with aninteraction between the ultrasonic standing wave and a target object;and means for generating an image of the target object based on thedetected change in the one or more characteristics of the ultrasonicstanding wave.
 26. The ultrasonic sensor system of claim 25, wherein theduration is based on a number of the excitation signal pulses.
 27. Theultrasonic sensor system of claim 25, wherein detecting the change inthe one or more characteristics of the ultrasonic standing wave includesdetecting a change in amplitude and/or phase of the ultrasonic standingwave.
 28. A non-transitory computer-readable storage medium, wherein thenon-transitory computer-readable storage medium comprises instructionsexecutable by a processor to: apply a plurality of excitation signalpulses to an ultrasonic transmitter of an ultrasonic sensor unit,wherein a frequency of the plurality of excitation signal pulses isselected to generate an ultrasonic standing wave inside a cover glasslayer of the stack of material layers, the cover glass layer configuredfor contact with a target object, and wherein the plurality ofexcitation signal pulses are applied for a duration to allow buildup ofenergy for the ultrasonic standing wave over a first threshold level;detect a change in one or more characteristics of the ultrasonicstanding wave associated with an interaction between the ultrasonicstanding wave and the target object using an ultrasonic receiver of theultrasonic sensor unit; and generate the image of the target objectbased on the detected change in the one or more characteristics of theultrasonic standing wave.
 29. The non-transitory computer-readablestorage medium of claim 26, wherein the duration is based on passing ofa reference time period or on a number of the excitation signal pulses.30. The non-transitory computer-readable storage medium of claim 26,wherein the instructions to detect the change in the one or morecharacteristics of the ultrasonic standing wave includes instructions todetect a change in amplitude and/or phase of the ultrasonic standingwave.